Conventionally, a solder containing lead or a lead glass frit have been mainly used in bonding and sealing of oxides such as glass, etc. around 380° C. However, lead has been inadmissible to use due to environmental problems. On the other hand, while reviewing various brazing filler metals, brazing sheet, etc. described in “JIS Handbook (3) Nonferrous metal”, it is difficult to find such a material as having a melting point at or below 400° C., having good adhesion properties, and causing no shrinkage crack on glass due to a difference in coefficient of thermal expansion between glass and the brazing filler metal.
The inventors of the application have recently proposed a solder alloy for bonding oxides, which contains Sn as a main component and to which Ag and Al are added (see WO2007-007840).
As a solder alloy having a low melting point and containing no lead, the solder alloy proposed in WO2007-007840 has an excellent bonding strength and an airtight sealing performance for oxide materials such as glass, ceramic, etc.
However, in bonding with the solder, a surface of the molten solder is oxidized and an oxide is generated in some cases. Since the oxide does not contribute to bonding, it results in a decrease in bonding strength and airtight sealing performance.
Also, when gases are generated in a solder or taken in from the atmosphere, the gases expand in the molten solder during heating after bonding, and sometimes voids generate in the solder. In particular, when heating and sealing in vacuum are carried, there is caused a problem that gases in voids are liable to expand to large voids so as to damage bonding strength and airtight sealing performance. There is also caused a problem that bonding strength is decreased, and results in generation of leakage in the case where a large amount of oxide is generated on bonded interfaces at the time of bonding.